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How do you write 'to', 'then', or 'via' in IFR clearances shorthand symbols?
Although there are NO restrictions about the usage of shorthand symbols, still it would be useful when I fly with another person together, so I studied those symbols and got some doubt points.
If someone wants to write 'N123 is cleared to AAA airport'
I assume it would be 'N 123 C AAA A', so shall I exclude 'to' in this sentence?
How about 'via B departure then as filed'?
It seems that I can write 'via' as 'VIA', But I can't replace 'then' with '>' or '→', since each of those means 'before' and 'cruise'.
I referred to the site below, about symbols :
https://nova.aero/repository/IFR/IFR%20Clearance%20Shorthand%20Symbols.pdf

do you mean 'via B departure then as filed'?

Sorry, I changed that..

Do what works for you, and omit what you can. For example, "N123 is cleared to KABC" can be condensed to "ABC"... assuming the clearance is for you, why copy your own tail #? You're copying a clearance, so "is cleared to" is, to me, entirely implied by the fact that there is something written.
"Climb & maintain 7,000" is for me an "M" with a horizontal line through it, and "7". If it was 5,500 I'd either write it as 55 or 5.5, since neither 550' nor 55,000' would make any sense. "Expect FL210 in 10 minutes" is "x 210 / 10" -- or "x21/10" -- because I know that I won't be cruising at 2,100', "21" isn't ambiguous.
"As filed" is AF, "climb via the (filed) SID is "via". Direct is a "D" with a horizontal line through it. (Or an arrow, but omitting the arrowhead saves writing & is equally clear that this is a symbol with specific meaning, rather than a letter.) Some people use "DCT" for direct, which is fine, although the D symbol will never be confused for being a navid I need to locate. "Right" and "Left" are circled R and L, if needed.
"Squawk 1234" is "sq 1234" and "departure on 126.55" is copied as just the frequency, unless there are several - then "dep 126.55 mtr 121.65 5 pr" would record the need to also call Metering 5 prior to start. If it's Ground or Clearance I need to call 5 prior, I'd probably just abbreviate those as G or CD, without the frequency unless there are more than one -- I have all of those on my EFB so "Gnd 121.9" is no more info for me than just "G".
For me, the recorded clearance isn't a document I need to refer back to months from now so I can recite verbatim what the controller read to me; it's a memory aid so that for the next hour or two I can know the pertinent elements without having to remember them. So numbers like squawks, frequencies, headings, and altitudes are vital. Fixes and procedure names likewise, which is easier with a printed copy of the filed route handy, where I can underline those elements I was given.
For example, if the filed route for KABC to KXYZ is to the ABC VOR, then to AFFIX then to BBFIX then V123 to DEF then the WHOZT1 arrival, the clearance might be "on departure fly heading 150, radar vectors to BBFIX then as filed," so I'd write "H 150 RV" then underline BBFIX, and below that I'd write "AF". After I'm done reading that back, I'd line out the ABC VOR and AFFIX (that weren't part of the issued clearance).
Your instructor may have a different way that's preferred, and in that case go with what you're being taught. But over time, this is what I've found works well for me. Less writing, no need to scramble, what I need & no excess when I come back to look at it later.

In Canada we use FPR, Flight Planned Route, when the clearance was as-filed.

You may find it useful in the early stages to use a template with data blocks for writing your clearance instead of trying to copy everything said free hand.  There are commercial options available, but it isn't difficult to create one yourself with a fill-in-the-blank format.
For example, write or type out and print up a kneeboard card with: "N123Y is cleared to _____ via _____, then _____.  Climb and maintain ______, expect _____ , ______ minutes after takeoff.  Contact departure frequency ______, squawk _____."
If you do this somewhat regularly you will find that in a very short while you can ditch the template, write down just the information in the blanks, and your mind will fill in the rest.
The information follows a pattern so is isn't difficult to decipher as long as you write it in the correct order.  For example, try to decode the following clearance data without any transitional words:
PAE
Tim1 SID,
D SEA, AF.
2000'
5000' 10 min,
125.7,
4775
Not too difficult I presume?

Whatever abbreviations/symbols you use are only relevant to YOU, nobody else needs to be able to read them, nobody else needs to approve them.
The clearance will always follow the RAFT format, so write down the letters R A F T in a column on your pad, and then fill in  what you're told on the appropriate lines, using as little writing as possible.

At least in the US, the standard mnemonic is CRAFT, not RAFT, but that might be different elsewhere.


text:
"Snowglobe"-type attitude indicator
I'm an aviation enthusiast (I do not, nor have I ever worked in the industry), and after researching several various emergencies (reading text documents, reviewing CVR transcripts and watching documentaries and TV shows such as Air Crash Disaster/Mayday), I'm curious as to why so many spatial disorientation situations have resulted in the loss of aircraft.
My thinking is that even if there is zero visibility combined with possible instrument failure causing disorientation, why don't aircraft have a simple glass ball half-filled with water set atop the console?
Wouldn't that provide a crude but reasonably accurate aircraft attitude? I mean:

water is at the top of the ball (ie. toward the ceiling), plane is inverted
water is angled toward the left of the ball, plane is banking left (similarly for right)
water is angled forward, plane is nose down
water is angled aft, plane is nose up

Even if a/both pilot(s) were fully disoriented due to whatever situation to the point they believe their attitude and other instruments aren't jiving with what their balance is telling them, a quick look at such a simple device would at least indicate a reasonable measure of sanity, wouldn't it?
Now, water might be too viscous a fluid for this, but replace with an oil or similar to minimize movement due to vibration etc.

https://wonders.physics.wisc.edu/water-pail/   In flight, acceleration feels the same as gravity, and can appear in any direction, including completely sideways or upside-down from true gravity.  A globe of water is subject to acceleration.  If the plane does an abrupt loop, the water will maintain its position in the globe, even as the globe turns upside down! (like swinging a bucket over your head).  The only known way to measure attitude is a gyroscope (and related variants, MEMS, laser-ring)

@abelenky Indeed, and it's totally counter-intuitive. That's why it is so dangerous to trust your guts when there's no actual vision and also the reason why flight simulators work and can give the impression of basically all attitudes even though they are fixed to the ground.

This video demonstrates clearly why your idea would not work:  https://www.youtube.com/watch?v=V9pvG_ZSnCc

FAA studies have shown that non-IFR rated pilots last about 2 minutes before losing control when instrument conditions develop (or when they fly into clouds, or into very low visibility conditions).  2 minutes, that is not long.  Part of our training is to keep the plane at same altitude and perform a 180 degree turn to get out of the IMC condition. Only takes 1 minute to perform a 180 degree turn at standard rate of turn. If the plane is not equipped with a turn coordinator or attitude indicator/artificial horizon, the standard bank angle can be difficult to determine, ->> loss of control.

Thanks everyone for clearing this up. The bucket/water test, which I'm sure we've all done in childhood (and even in adulthood) pretty well explains it all.

"My thinking is that even if there is zero visibility combined with possible instrument failure causing disorientation,"  IFR equipped planes also have several means to determine if they are making uncommanded turns: the artificial horizon/attiude indicator (vacuum gyro, or solid state these days), the turn coordinator gyro (usually an electric gyro), the whiskey compass or vertical card compass, the directional gyro/Horizontal Situaiton Indicator (HSI) (vacuum gyro, or solid state these days). There's 4, moving map GPS would be another indicator. Proper train and practice help too.

Thanks @CrossRoads, you've given me some new instruments to research. I am familiar with basic gyros due to my embedded microconroller development I do, but I wasn't aware that there were so many various ones related to flight stability.

I have a pair of Garmin G5s in my plane, attitude indicator on top and direction gyro/HSI on the bottom, and a STEC-20 autopilot/turn coordinator.  The top G5 also shows many things that the pitot/static system shows, but I'm just getting used to reading those on the G5 vs getting the info off the old guages: airspeed, altitude, vertical speed. Even has turn coordination, heading and ground speed.

The heading info comes from a Garmin GMU11 magnetometer, so I don't even look at my compass much anymore either, and the heading doesn't precess and need resetting every so often like the vacuum direction gyro did.

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What's parked in Mil Moscow helicopter plant?
This answer has a photo of a Mil Mi-26 that I felt in love with, so I looked in Wikipedia and learned it's the largest production helicopter, second only to Mil V-12 that flew as a prototype.
In turn, Wikipedia's V-12 article gave a link to the exact position where one of the two V-12 produced is placed, so I opened Google Maps and I had a satellite photo of the V-12!
So far so good, but when I zoomed out a little I saw many other helicopters parked around, all clearly helicopters except a cigar shaped beast, with winglets apparently too small for an airplane, in the upper right corner of the screen dump below, marked with an arrow... Remarkably I cannot see the shadows of the blades on the ground.
Could you please identify that object?

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Is there any agency or meeting, at global level, that rules flight zone interdictions?
Considering the number of conflicts in the world, and the tragic event of the airliner crash over Ukrainia caused by military action, there are a lot of flight interdiction zones on the world.
From the data I gathered, those interdictions are pronounced by regional agencies, such as the FAA for the USA or the ESA for Europe. However, some countries like United Arab Emirates or Qatar have airline companies, but I don't think they have agencies such as the FAA. Still, do airplanes for those companues fly over East Ukraine?
So the question is:

Is there any global agency that coordinates the questions of dangerous flight zones?
Is it able to enforce an interdiction, or does it just put up meetings to coordinate the FAA, ESA and airline companies?

Related: Who decided which airspaces not to fly through and how? and Where can a realtime map showing all flights and all dangerous areas be found

I will ask again the question since in the other question's answer, only country authorities are mentionned and not FAA, ESA or similar ones

Instead of closing, I think just answering "no there are not such global authorities" would have been good :)

Nit: Every country has a Civil Aviation Authority, usually called something similar to that generic term in the national language.


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Why so many designs of Chinese carrier-based fighter aircraft?
I'm quite confused about all the carrier-based fighter aircraft projects in China. Could you help me determine which are the different acronyms (same of different?).
According to wikipedia, there are three possible aircrafts aboard the most recent Chinese air carrier:

Shenyang J-15, which is already in service aboard Liaoning
Chengdu J-20
Shenayang J-31

But looking in the internet, I also found the J-21 that looks like an extension of the J-20. And there is also the newly shown, stealthy, J-35.
Are all those designs similar? What are the differences and the advantages of each of them, since having three diffferent types of aircraft aboard a single carrier is not good idea (speaking about maintenance)?

Did you know the Ford class aircraft carriers carry more than 8 types of aircraft?

@RonBeyer I did not know and fast check on internet does not give the type of aircraft. Is it 8 types of fighter aircraft or do you include recon, strike...?

I'm not sure what you're really asking here. The J-15 is a reverse-engineered SU-33, ie a 4 gen fighter which is expected to be in use until the J-20 and/or J-31 (both are 5th gen) will be available. If you are asking for specific differences between the 3, then skimming through the wikipedia articles on each should provide more than enough information (which I presume any one that will attempt to answer this question will do anyway)

Your question didn't focus on fighter aircraft, as far as I know, the Ford can handle all Navy aircraft including the F-18, E/A-18, and F-35.

@RonBeyer, from a launch and recovery standpoint the 18s might as well be considered a common airframe.  So I’m struggling to find eight.

@MichaelHall by that logic C-2 and E-2 would also be pretty much one and the same

@Jpe61 - No, those two are completely different airframes.  The E/A and E/F are different only in the equipment they carry and their mission.  Regardless, with the demise of the Tomcat, Viking, Prowler and Intruder, carrier air wings have become very homogenous in appearance.  And I can’t come up with eight…

@MichaelHall i thought you were referring to Hornet vs. Super Hornet. C-2 and E-2 share wings & powerplants, prolly other parts and systems also, and from launch and recovery standpoint they are quite similar.

@Jpe61, yeah, sorry I didn't make it more clear.  I can't fathom why the Navy kept the 18 designation for a completely different airframe that is only visually similar to its predecessor.  But, it may be useful to share what carriers consider "similar", at least from a recovery standpoint.  There are 3 critical parameters that need to be met to safely catch an airplane: 1. Arresting gear setting for the max landing weight; 2. Minimum recovery headwind; 3. Lens setting to accommodate airframe differences in hook-to-eye distance.  Between the E2 and C2, all three parameters are different.

@RonBeyer I edited to make it clearer: it focuses of on fighter aircraft of course since I gave examples of fighter aircraft only

@Jpe61 the point for Ford-class is that F-18 is for fighting, F-18 super hornet is the new generation and is supposed to replace F-18, EF-18 is for electronic warfare, and F-35 is like "breaking through" a generation. So the fact that there are in the carrier group is explained. For CHinese examples, similare reasons could apply but the actual use on a carrrier is not sure given parcellar information available

First, the J-15 is a 4th generation multirole fighter, the western counterpart for it woud be the F/A 18E. While it is still in production and fully capable, it lacks, first and foremost, the stealth properties of 5th gen fighters such as the J-20 and the J-31. The latter two being considerably more modern can be assumed to generally outperform the J-15 by a considerable margin.
The distinction between J-20 and J-31 is their primary roles: J-20 is an air supremacy fighter such as F-22, while J-31 is designed as a multirole fighter, such as the F-35.

Ok thank you for the explanation of differences. Are they all planned to enter carrier service? Because for example the F-22 is not on carrier service

It is my understanding that current J-20 is not carrier capable. I gathered by the info supplied by you in the question that it might be

Space is a premium on a carrier so a multi-role fighter is much preferred than single role J-20. This is why Russia retired Su-33 and US retired F-14. So from the very beginning J-20 wasn't designed to be carrier-capable like F-35 or F-14 was. As for J-31 I think it's more for the politics: deploying J-31 on carriers will cause the neighbors to deploy lots of F-35 to counter-balance, which isn't a favorable outcome for China.


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Limitations of bird flight as an aircraft design
What is it that stops man from duplicating Bird Flight as Engineering?
After all, birds do not require separate propullsion! They use the same wings for propulsion and navigation.
My question could be broad. Is it far-fetched ?
With solar energy harnessed and electronic control systems, SHOULD we not take a re-look at bird flight, at least as personal carriers or even as a short-haul option ?

"After all, birds do not require separate propullsion!" They have a complex muscular system that provides the additional propulsion. We can mimic that in devices the size of birds, but when we scale up to the size of aircraft, moving wings like that becomes a major (power) problem. It is easier (aka fuel efficient) to pull it through the air via engines rather than try to flap 300 feet of wings. Not to mention the up/down motion of the body would be very uncomfortable for passengers.

It's actively researched, not for propulsion, but for controls.

Antzi , your reply is encouraging. Maybe , the controls could reduce inefficiencies in power usage.
After all, we do not always need a multi-seater airliner. Even a 10-kg carrier for home-delivery of groceries or restaurant deliveries could be looked at.

Birds have a tail that's used for turning and stability. So they do have separate controls :)

jwenting , not discouraging !

Have a look at this: https://en.wikipedia.org/wiki/List_of_largest_birds Notice how the largest flight-capable birds are not far over your weight target, and keep in mind that these devote most of their mass to the purpose of flight, that is, they can carry almost no payload. Further notice that most of the largest examples are soaring birds, dependent on updrafts for endurance.

See "Bird Flight As The Basis Of Aviation", Lilienthal.

Very similar question on the issue of idea of using flapping wings instead of engines: https://aviation.stackexchange.com/questions/17133/can-planes-flap-their-wings-like-birds  If that doesn't answer your question, please help us understand what you're looking for.

Possible duplicate of Can planes flap their wings like birds?

@RonBeyer That comment looks like the core of an answer.

@RahulTandon Not enough to be an answer, but this is similar to asking why don't cars use legs? We can make them do so, but a wheel is much more efficient. Perhaps a different look on this problem is why don't birds use propellers? Squared cube law and the complexity of living things plays into both of these design choices.

It has been looked at. This class of aircraft is called an ornithopter. Scale model ornithopters can be made to work, manned ornithopters are few and far between.
A big problem with a large ornithopter is that you're replacing a static structure (the wing) with a large, reciprocating mass.

The structure gets a lot heavier and each joint is a point of failure.
you have to convert the high-speed rotating motion of the engines to low-speed linear motion, which means (heavy) hydraulics and large losses
imagine the complexity of a wing that's articulated, and at the same time needs to be able to change its profile multiple times per second at each point of its span. You soon end up with an aircraft that can't carry anything because the wing is stuffed with hydraulics.
on a bird, the wings are a large fraction of the total area and weight of the bird. It has a small payload. Scaling up the concept only makes this worse (square/cube law).

+1 for ornithopter. I hope developing can go forward for very low and slow plus high reliable recreational uses, akin to Hang gliding

The movement of the bird wing during the flight is very complex. Not just the trajectory matters, the shape of the wing itself changes as it moves. Angle of attack changes continuously. It is definitely not just about flapping up and down. For a long time, this limited the possibilities of building such machines, while with recent technology it seems possible.
This complex movement also restricts the maximum speed possible, as even small birds cannot beat the wings more than about 80 times per second. The fastest birds, such as the white-throated needletail, may reach 105 mph (168 km/h) in horizontal flight that would not be impressive for an aircraft.

Add to that the inherent instability of birds which requires them to adjust their wings fore and aft all the time.

Much of the early history of attempted flight is about failure inspired by bird flight. It wasn't until the idea of flying stopped trying to incorporate bird-like flapping movements that it started to see any kind of success.
Wing-propulsion is a dead-end for human flight.
In fact, for large birds, wing-propulsion is also not as efficient as it is for smaller birds, because of the way that scaling works - if you double a length, you get four times the area, and eight times the volume (and mass).
Larger birds tend to spend most of their flight soaring, making use of the movement of air itself - and many of them, far from flapping, are able to lock their wings into place.
There are many aspects of bird flight that have successfully informed human flight, but using wings for propulsion is not one of them.

For the use of wings for both propulsion and sustaining flight, it has already been done as ornithopter
For the use of flexible wing for control, it is currently under research. For example, I can cite the NASA dynamic Control Surfaces and Active Wing Shaping Control.
For the solar energy, I may add that gliders and birds use thermals.


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Instructor can't talk when H10-13.4 is plugged in
I'm a student pilot and I just bought my first own headset because the standard one in the plane does not really fit me.
I got a david clark h10-13.4.
My instructor uses either his private Bose A20 or the standard (active) Aerostar that is in the plane.
Today I tried my DC-headset for the fist time and there was a problem: I could talk just fine and the mic worked great but I could not hear my instructor anymore. He had to yell into the microphone to activate the intercom and even when he was loud enough, the sound quality was aweful. This was the same for both the A20 and the Aerostar headset.
Also others could not hear him when he tried to communicate with other aircraft over radio. They said there was just noise transmitted.
As soon as I switched back to the Aerostar headset, everything was fine again.
After today's flights I tried to use my headset with a handheld radio and the quality of my transmission was good (both ways).
My first idea is an impedance- or level-mismatch; but I thought, all fixed-wing  aviation headsets had about the same impedance.
Are there ready-built circuits to adapt my headset to the intercom?
I really don't want to stick with the standard headset. Neither do I want to blow 1k€ on a Bose as I really like the H10.
Apart from it being a Becker I don't know anything about the radio in the plane but I've been told not to mess with the configuration of the radio, as it works in all other setups.
Edit:
After having found a datasheet for both the A20 and the H10-13.4 I'm more confused than before.
DC-Headset-Datasheet: http://www.davidclarkcompany.com/files/literature/10-13.4.pdf
A20-Datasheet: https://assets.bose.com/content/dam/Bose_DAM/Web/consumer_electronics/global/products/headphones/a20_aviation_headset_Orion/pdf/a20_hdst_PDF_ownersguide_EN.pdf
On page 38 of the A20's datasheet there is the technical data with the microphone output levels. It sais:

Typical output is 600 mV at 114 dB SPL

The David Clark sais:

Mic Sensitivity: 400mV+/-6dB into 150Ω load @ 1kHz for 114 dB SPL input

which seems to be tested under the same conditions. But the amplitude of the Bose is higher. Shouldn't the effect we have seen be the other way around? With his mic working and mine not activating the intercom? Or with him being very distorted due to clipping in the amplifier?
Hope someone knows better than me.

Becker? Is it a combo radio-intercom? What is the model?

Thank you for trying to help, Juan. From a photo Ive taken in the plane it seems to be this radio with integrated intercom https://www.becker-avionics.com/portfolio/ar6201-vhf-transceiver/. I will be at the school for checking only by the end of the week. I will update the question then

The manual has this note:
"At a setting for VOX threshold of -15 a convenient behaviour of the VOX should be achieved in most aircraft. This requires that mike sensitivity had been correct adjusted (Configuration Settings*). If the mike sensitivity is not correct adjusted, VOX may not work satisfying."
This sounds like the likely culprit.

So after Juan's help and also talking to Becker I think I've found the answer:
It's an unlucky impedance combination.
That means maybe I can get it to work by lowering the VOX-level (increase mic bias) far enough so my instructor can activate it but if I lower it too much, my own mic will constantly activate the intercom, resulting in constant noise.
Becker told me, this is likely due to an old wiring version being installed where both pilot and copilot's mic are connected in parallel to the same amplifier. So when my mic has a 'better efficiency' i.e. better conversion from mechanical swing to voltage, the VOX adapts the threshold (or as Becker said the bias voltage for the mic) for this and the amplitude of the co's mic is to low to activate it.
A fix would at least require rewiring of the system to have dedicated amplifiers and bias supplies for both the pilot and co-pilot.
I think I confused some things in the physics here; if you point me to it, I will gladly correct it.
In general Becker's advise was to always use the same kind of headset for both pilot and all passengers in order to ensure best results (all volumes will be the same for all people then).


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Why does instability in pitch lower trim drag for an aircraft with a tail?
I have a hard time figuring out why a longitudinally unstable aircraft has a lower trim drag. This seems to be a thing especially in fighter aircraft, so does it have to do anything with supersonic flight as well?

There seems to be a lot of confusion on this issue.  Perhaps if you share your sources of information I might be able to give you a better answer.

Well that's the thing, I don't have any sources. I actually stumbled upon it during my exam preparations, it's an old exam question but no answer are provided. I added an image from the lecture materials though.

The aircraft is only unstable in subsonic flight. In supersonic flight it is stable again, but much less stable than a subsonically stable version. Since stability costs trim drag, supersonic trim drag is greatly reduced. As you correctly note, there is no drag reduction in subsonic flight. However, an unstable layout allows to reduce wing and tail area, so there is some drag reduction indeed.

It might be helpful to look at this article from Boldmethod.
The basic idea is that the farther away the center of gravity is from the center of pressure (or center of lift depending on your terminology) the more lift the horizontal stabilizer will have to generate.  The more lift an airfoil has to generate will result in more induced drag.  Trim drag is specifically the induced drag created by the horizontal stabilizer.  The horizontally stabilizer can actually produce both positive (upward) and negative (downward) lift but the negative lift has the even worse affect of requiring the wings to create more lift to compensate for the downward lift as demonstrated in the Boldmethod article.
Having the center of gravity at the center of pressure also allows the aircraft to be more maneuverable since less force is required from the horizontal stabilizer to initiate a maneuver.  This is why fighter jets tend to be longitudinally unstable and use fly-by-wire to compensate for it.

This is why fighter jets tend to be longitudinally unstable - Really? If it were as you say, they should be indifferent. Instability will again cause trim drag, just as the answer supposes.

I though the longitudinal instability of fighters was to be more reactive in pitch.

Longitudinal trim is achieved when the total pitching moment on the aircraft is zero. Except for some special cases, the tail (or elevons in the case of tailless aircraft) will generate some lift to trim out the pitching moment from the wingbody. Therefore, if the tail generates negative lift (thereby providing a nose up moment), the wingbody needs to work harder (i.e. higher AOA) to generate the aircraft level $C_L$ required for level flight.
At this point, let's define what trim drag actually is. Trim drag is the ensemble of: additional induced drag from tail incidence (or elevators or elevons), plus additional induced drag from the wingbody due to a higher AOA required to achieve the total $C_L$, plus additional interference/viscous drag due to control surface deflections. The first and third components are actually relatively small compared to the second component. Unless you have a big control surface deflection, the majority of the trim drag actually comes from the loss of lift!
When the aircraft is pitch stable and trimmed, there is a reduction of $C_{L_{\alpha}}$ compared to the untrimmed case. As the static margin is reduced, amount of negative lift from the tail is reduced and the lift slope improves. This also improves the trim drag.
The following graphs illustrate the effect of static margin with trimmed $C_{L}$, amount of h-stab needed to trim and trim drag, generated with typical aircraft geometries and aerodynamics:

Excellent graphs.  Folks should remember with negative static stability there will be an increase in drag due to pitch oscillations (which can be computer controlled) and an increased danger of stalling the aircraft.  Static stability (speed stable) is a valuable safety factor to pilot under high workload, as the aircraft can be trimmed to hold speed and AOA.  A staticly unstable aircraft is much more dangerous in slow flight.

Different question, same answer. Active stability allows for the $c.g.$ to be behind the centre of lift, thereby compensating for the associated aerodynamic instability.

For passive longitudinal static stability (by aeroforces), the total centre of lift $C_N$ must be behind the centre of gravity. At all angles of attack and all velocities, with stalled wings etc. The only passive solution that is always safe in all circumstances, is the aerodynamic centre $a.c._w$ behind the $c.g.$, always creating a nose down moment which must then be compensated by an aerodynamic nose up moment from the tailplane: negative lift. So we need to compensate this by more lift from the main wing, with associated induced drag.
That's the passive, aerodynamic solution. If we allow $n.p._{fixed}$ to be in front of the $c.g.$, the tailplane will always help in creating lift, not destroy it. At cruise, we can trim the aeroplane for neutral pitching moment, but if there is a disturbance in angle of attack (like a vertical gust) the main wing will create more lift than the tailplane (it makes sense to make the main wing the most efficient one.) But that means that any disturbance in AoA will create a sudden, unstabilised nose up reaction: static instability.
The only solution to be able to use the $n.p._{fixed}$ before $c.g.$ situation, is by using active stability. Any disturbance in pitching moment is immediately counteracted by an automatic elevator deflection, like balancing a stick vertically on an open palm, or riding a unicycle.
This principle goes for both subsonic and supersonic flight. But going supersonic means that the Centre of Pressure shifts rearwards: Mach tuck. The aeroplane could be:

passive statically stable in supersonic flight, unstable in subsonic flight.
passive statically unstable in supersonic flight, and much more unstable in subsonic flight.

These answer seems to be at odds w/ the fact that many old-fashioned free-flight model airplanes clearly had lifting tails.

@quietflyer the drawing shows a statically stable configuration with a lifting tail.

You can be staticly stable with a lifting tail but still get into trouble if the rear wing stalls first.  A solution is to use a straight front/delta rear.  This was a very popular combination for fighter biplanes, but totally unnecessary for an airliner.  But it is understandable that these behemoths would be extremely sluggish if TOO stable.  But too unstable is unsafe.

It's really simple. To hold the aircraft up in the air, the TOTAL Sum of Up Lift and Down lift (plus or minus) must be equal to, and oppose, the weight of the aircraft. If the lift from the tail is pulling the aircraft down, towards the earth, (as in a positively static stable aircraft), then the lift from the wing must be higher (by twice the tail amount pulling down) to counteract it. The total lift up minus the total lift down must be equal to the aircraft weight. So since the total lift Up and Down are both producing drag, then there must be more drag.
  Say the aircraft weighs 1000 pounds and say that drag is 10% of total lift
          Wing Lift      tail Lift     Result       Total lift    Drag
 Stable    1200 lbs      -200 lbs      1000 lbs      1400 lbs    140 lbs
 Unstable  800 lbs        200 lbs      1000 lbs      1000 lbs    100 lbs

You may wish to express tail force in torque as -200 ft lbs (as it would have a longer lever arm than the wing).

True, but this issue is not related to the moment (torque) it is simply related to the force the aerodynamic surface is exerting on the airframe. The tail is pulling the aircraft down in an aircraft with positive static stability, and it is pulling the airframe up in one with negative static stability. True, these forces generate moments, but it is simply the force being exerted that explains why the trim drag is higher in one case than in the other.

"The tail is pulling the aircraft down in an aircraft with positive static stability, and it is pulling the airframe up in one with negative static stability. " -- I suspect this is not true.  A frequent topic of discussion on this site.  I don't think it's always true that a download on tail is required for positive static stability.

See for example some of the comments under here- -https://aviation.stackexchange.com/questions/66311/does-a-tandem-ultralight-need-flaps-on-the-rear-wing-if-the-front-wing-has-flaps?noredirect=1#comment173407_66311

@quietflye, yes you are right, it is not ALWAYS the case. Canard aircraft, for example, violate that premise. They can have positive stability with both the canard and the main wing (aft) generating positive lift. But generally, in a conventional aircraft, with a aft mounted tail, if the CG is forward of the AC, BY DEFINITION, the sum of all aerodynamic forces (which BY DEFINITION can be thought of as acting through the AC, will cause a nose down pitching moment. Obviously, to counteract this, a tail mounted elevator is AFT of the CG, and must exert downwards lift to generate a nose up moment.

@CharlesBretana That's not quite accurate. Wing aerodynamic forces act through the center of pressure, not wing AC. With a conventional aircraft, if the static margin is sufficiently small (e.g. 5%), the tail can generate positive lift for quite a large range of speeds. The absolute lift from the tail is not a good indicator of static stability.

Actually, not. all of this gobbledegook is just engineering convenience. In reality, all aerodynamic forces push on the airframe at each individual piece of the surface of the airframe, with a force equal to the normal pressure at that individual point, in a direction exactly normal to the surface at that point. All other concepts, CoP, AC, whatever, are just mathematical conveniences to allow engineers or academicians to perform calculations (or simplify explanations).

Think about it... One definition of "Center of Pressure" is "The center of pressure is the point where the total sum of a pressure field acts on a body". What exactly does this mean? What is the "total sum of a pressure field"? This does not exist! It is only the abstract result of a mathematical vector operation on the actual trillions of individual forces created by the impact of each air molecule on the airframe surface.

… and the definition of the Aerodynamic Center includes the concept of a "moment", which is the resultant torque about a point, from a force acting on a body. Again, there is no single force, there are only the trillions of individual forces from all the molecular impacts. The way engineers think about is an (accurate, and equivalent) abstraction. But even so, it is based on the same forces that the definition of CoP assumes is "Acting through" a different point!

All the exceptions to my statement that you mention only exist because these concepts, in attempting to "simplify" the actuality, ignore the aerodynamic forces from all the other parts of the airframe surface. If the rest of the airframe, (other than the wing and tail surface), after calculating the contributions from the CoP and/or the AC, contribute sufficient nose up moment to both counteract the nose down moment from the wing's contribution (from a very small "wing" static margin)then the resultant total might require the tail to contribute a nose down moment to balance.

@CharlesBretana The point where the total sum of pressure field acts on a body most definitely exists. It is no different than saying the center of mass exists. The aerodynamic center, however, is indeed an approximation. If the CG is vertically offset from the MAC, then AC does not exist. I'm not talking about wing static margin; we aren't dealing with flying wing here. SM is in reference to the neutral point. 5% ahead of wing AC would actually be quite a lot of SM.

@CharlesBretana I'm happy to show you the maths, using parameters that quite closely mirror typical low Mach aircraft. I would suggest we either take this to chat or start another question.

Of course the point exists. And of course it is exactly the same as saying that the"point" calculated to be the CG actually exists. What's wrong is to believe that all the mass is actually at that point, just it's wrong to believe that all the aerodynamic forces actually act through the AC or through the Cop. These assumptions are only valid for certain specialized applications. For others, they are wrong. For example, when performing the calculation to determine the gravitational force inside of a spherical body.

And I'm not saying any of these things are only "approximations" . indeed, to the degree that the values used to calculate them are exact and accurate, they are also exact and accurate. All I'm saying is that they do not reflect what is actually happening. They are engineering abstractions exactly analogous to using imaginary numbers and complex variable functions to calculate solutions to aerodynamic PDEs (Navier Stokes) equations.

Let us continue this discussion in chat.

You need a picture of a 3rd airplane with the center of gravity right underneath the wing lift arrow.  Notice the elevons would be neither up or down.  This is the lowest drag configuration, as up or down elevons adds drag.  Up elevons (staticly stable) are slightly more draggy (because they force the wing to work harder against their downforce) than down elevons (staticly unstable), but both are more draggy than no elevon deflection.
IMPORTANT TO DESIGN:
It is the horizontal stabilizers job to set optimal AOA of the single wing (since the 1920s) while the Hstab is at 0 (lowest drag) angle of attack in flight.  The difference in incidence is called DECALAGE of the horizontal stabilizer (See B-52).  The center of gravity optimally belongs DIRECTLY UNDER the center of lift.  The horizontal stabilizer must have ADEQUATE VOLUME to hold that wing in place.
Next one decides how much static stability (speed stability) one wants in the plane for safety.  This is dependent on weight placement of fuel and payload, as well as potential shifts in CP due to change in AOA and ENGINE THRUST torque factors.  Without active stability control (computer control) this is generally set up to be positive.
Setting up for static stability and trimming aerodynamicly is usually accomplished with a small trim tab.  Sadly, this seems to have lead to a belief among modern designers that a tiny horizontal tail volume is OK, and computers will solve everything.
We even have graphs showing us that creating a staticly unstable tandem (biplane) will save fuel.
Starting every morning by looking at a Piper Cub may be helpful.


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Why does indicated stall speed change?
Aviation confusing me... I’ve read that stall speed doesn’t change (IAS) no matter what altitude you’re flying - of course under specific conditions ISA, 1G level flight, no wind, gross weight etc. then why I keep witnessing for example in the Boeing 737-800 PFD (primary flight display, intend to the barber pole) that at low altitude let’s say your stalling speed is around 140CAS when at cruise level stalling speed is way above - approximately 220CAS (random number) Why is that ? Doesn’t stalling speed must to be the same at all altitudes?
Thank you all for your answers!

I don’t have enough time for a proper answer now, but google: Equivalent Airspeed vs Indicated vs True

It happens because of a compressibility error in the airspeed indicator (yes, even in the digital ones, since the error is not mechanical, but a physical property of the air).
As you might be aware, the speed indicated on your instruments is not really a speed at all, it is actually a pressure. Your pitot measures a deltaP between static and pitot and displays that pressure difference on a scale noted in knots.. we call that Indicated Airspeed.
This indication, because of changes of density, can be quite a bit different from your actual True Airspeed, but people didn’t care all that much about this difference, since the way the wing flies is reliant on that deltaP pressure difference anyway, so that you will actually stall at the same Indicated Airspeed, but not at same True Airspeed. (there is less density higher up but you are now traveling a bit faster, so the effect cancels out and the wing will behave the same)
The instrument was kept as it is in the cockpit (even though today we call it Calibrated Airspeed after a few changes) as a very usefull indication to the pilots.
Ok, now back to the question: turns out when planes begun flying faster and faster, a second error, caused by compressibility appears, that makes the airspeed indicator over-read. The pitot pressure is reported higher than it should be because at high speeds the air compresses in the pitot and makes the pressure there artificially higher, the deltaP is higher, thus our Indicated Airspeed is higher. Of course, this pitot tube phenomenon is not happening on the wing, so now we have a problem: we have a high speed indicated at the instrument, but most of it is actually just compressed air. Our Equivalent Airspeed is in fact much lower. And the wing will in fact always stall at the same Equivalent Airspeed. That means your Indicated Stall Speed in the cockpit will be ever higher as you climb because of compressibility.
Now, instruments were still kept to display IAS instead of the EAS, and a provision has been made where the stall barber pole advances up as you climb to make up for compressibility error
Then why haven’t the instruments been changed to show EAS directly? That is a harder question to answer definitively. I suppose by now people were already too used to having IAS in the cockpit, and the error is only really a factor for jet planes flying above (say) + 20 000 feet at speeds above Mach 0.5.

The difference is the change in Mach number over altitude.
And it is more than just the compressibility error in the IAS indication.
The maximum lift coefficient of a wing goes down with Mach number. While at sea level and 140 KIAS you fly at 21% of the speed of sound (Mach 0.21), at cruise altitude (I guess that means 30,000 ft) the true speed is already 360 KTAS which -- together with the decline of the speed of sound at lower temperature -- translates to Mach 0.63.
In order to estimate the change in maximum lift coefficient, look at the factor maximum lift coefficient times Mach squared: Above maybe Mach 0.4 to 0.5,  this is what should stay (roughly) constant. A typical value for a modern wing would be 0.4, so we divide this by 0.63² = 0.397. Thus your maximum lift coefficient at Mach 0.63 has dropped to about 1.0. At lower altitude the maximum lift coefficient of the clean wing is closer to 1.6.
Technically, the wing might even be able to create higher lift coefficients at Mach 0.63, but buffeting will make this intolerable. The stall speed at cruise level, therefore, is the buffet speed and cannot be directly compared with the (real) stall speed at sea level.

@ymb1: TAS goes up with the square root of the density ratio (1.225 at SL, 0.46 at 30 kft), and I took the 220 KIAS as the baseline (220 x 1.63 = 359). The c$_L \cdot Ma^2$ of 0.4 is my experience for a regular, mildly supercritical airfoil; with a given Mach number of 0.63 the lift coefficient must be 1.0 for the product to become 0.4. Not a silly question at all; I'm grateful for you checking my answers.

I'll assume you're talking about the barber pole presentation on the speed tape, flaps up at low altitude and flaps up at high altitude, with the pole moving higher on the tape at high altitude.  The barber pole indication and shaker firing point isn't related to the actual stall; it includes a computed safety margin that takes into account various factors like pitch motions and G loads.  That's why the barber pole moves around as you maneuver and pull pitch.
So the actual "indicated" stall speed doesn't change with altitude but what does change is the Stall Protection Computer's stick shaker (and pusher on airplanes that have them) trigger margins and barber pole indications, which have to allow for huge increase in inertial effects at high altitude (the mass is the same, but the air is thin).
The high you go, because of the magnified inertia effects relative to aerodynamic pressure (indicated speed)in the thin air, the more the Stall Protection Computer has to "lead" a change in angle of attack to provide a shaker trigger point that gives a decent margin above the actual aerodynamic stall.

The key of this question is the way the drag changes with speed and the maximum thrust the engine can develop at very high altitude, please read carefully the answer, it is a bit complicated, but I am confident
Let us first of all understand what is Vimd (Indicated minimum drag).
Said simply it is the speed of minimum drag, but let us understand a bit more
Drag has two components: induced drag and parasitic drag

induced drag is directly related to lift production and is greatest at low speeds and high angle of attack
parasitic drag increases in proportion to the square of the aircraft speed.

At any speed total drag is the sum of the above two components
For obvious economical consideration it is desired to fly with the lowest total drag which is also compatible with the desire of not using oversized engines
The following curve gives the graph of total drag versus speed

It appears that Vimd corresponds to the intersection point of the induced drag curve and parasitic drag curve
To answer the question

at low altitude let’s say your stalling speed is around 140K IAS and at cruise level stalling speed is approximately 220K IAS, Why is that?

For better understanding let us consider straightforward what happens at very high altitude.
When flying at high altitude, that is at an altitude where the engines thrust is limited by the low air density, and the pilot being therefore flying very closely to Vimd, if for a reason or another he reduces his speed, or his speed decreases, though still above the stalling speed, his actual engines thrust will rapidly become insufficient to overcome the increasing total drag increased by the speed reduction, as visible on the above curve. To maintain altitude he has to pull on the stick,( thus increasing the AOA and the drag), and simultaneously increase the thrust rapidly; if the thrust is not adequately increased, his speed will rapidly decrease and he might suddenly hit the real stalling speed. To avoid the above scenario the barber pole minimum speed is voluntarily well above the  stalling speed, to increase the pilots reactivity.
Normally at high altitude the autopilot and the auto throttle are engaged happily safely they look for the barber pole speed limit and not for the theoretical stalling speed.
the higher the altitude the larger the gap between the barber pole minimum speed and the theoretical stalling speed
Note, June 22, 2019. slightly edited for better comprehension.

I want to pitch in about 2 points:
First, Vimd you mentioned is the minimum drag air speed. For the jet aircrafts the minimum drag airspeed corresponds to max endurance airspeed. Thrust required at that airspeed is minimum, therefore fuel consumption is minimum. So you can stay in the air longest at that airspeed.
However jets do not use max endurance airspeed when they cruise. Instead they try to fly max range airspeed. Max range airspeed will provide minimum fuel consumption per miles flown. So you get places more efficiently.

Second, the part you mentioned here:
if for a reason or another he reduces his speed, or his speed decreases, though still above the stalling speed, his actual engines thrust will rapidly become insufficient to overcome the increasing total drag increased by the speed reduction, as visible on the above curve.

is not encountered during cruise flight as far as I know. It’s mostly an issue during the final approach when the aircraft is configured for landing. It’s called backside approach; meaning you are at the left side of drag curve.  “Speed stability” of the aircraft is unstable at that region.  Therefore requires more pilot attention and more throttle inputs to not go divergent.


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Are ILS CAT IIIA minimums based on the radar altimeter or on barometric altitude?

From what I understand the CAT II ILS minimums on the far right are based on radar altimeter 102'AGL and barometric altitude DA(H) 429'MSL (100'ATL).
But what about the CAT IIIA minimums in the middle? What is DH50' based on? Above threshold level or above ground level?

On the CAT II it says DA (decision altitude) 429ft. D(H) (decision height) (100ft). I think one can assume from this that the DH (decision height) for CAT IIIA is height above ground. But don't take my word for it, I only fly flight simulators as a hobby :)

Looks like it does.  "Decision Height" is a radio altitude value.  "Decision Altitude" is a barometric altitude value.


text:
What is the stall speed of the Antonov An-225?
Under certain circumstances - maximum gross weight, unaccelerated 1G flight, clean configuration (landing gear up and flaps)? Indicated airspeed (KIAS) of course.

We might need someone who can read the Russian POH to answer this.  The closest I found was a no-flaps landing speed of 295 km/h.  Flapped takeoff, at least 240 km/h.


text:
How does increasing the RPM result in a higher AOA for the blades?
I know that an increase in velocity (TAS) reduces the AOA and that makes sense. But how just simply increasing the blade rotation (RPM) results in higher AOA?

Following through on the thought, increasing rpm will (eventually) speed the plane up too, so the forward vector grows as well.  There for, the increase in prop AOA may only be temporary.  Interesting though.


text:
How does the aneroid wafer expands? (ASI)
So, how does it expand? (aneroid wafer - intend to the airspeed indicator) I have two “theories” (from what I’ve learned) which are :

The ram air entering through the pitot tube physically hitting, pushing the aneroid wafer forcing it to expand - giving us the actual indication in the cockpit. faster airspeed—> more air hitting, pushing  the aneroid wafer at higher velocity —> greater  expansion?
The ram air rapidly filling the aneroid wafer making the air more dense —> more pressure, therefore forcing it to expand due the pressure differential between the aneroid wafer and the sealed case, which contain the outside atmosphere pressure. Faster airspeed —> more air —> increasing pressure inside the aneroid wafer —> larger pressure differential—> greater expansion?

Which one is true ? or both maybe ?

Is there really a difference?  It's definitely a pressure thing, for sure.  Related to the dynamic pressure at the pitot tube.  Which could be construed as being a result of "physically hitting" on some scale.  Well, I'll let someone else tackle an actual answer on this one.

Check out this link:  https://allaboutairplanes.wordpress.com/2011/09/24/the-inner-workings-of-the-altimeter/

The aneroid wafer is just a metal balloon.  Add air into its interior, above ambient pressure, and it expands.
The outside of the wafer is exposed to ambient static pressure.  The inside of the wafer is plumbed to the pitot inlet.  Ram air into the pitot raises the pressure within the pitot plumbing to above ambient, which being connected to the interior of the wafer, inflates it.  The movement of the inflating wafer is geared to the airspeed indicator and voila, ya got yer speed indication.

The irony here is that aeronautical engineers build models of air flow based on a misapplication of data from pressure taps fed into the Bernoulli Equation to calculate airspeed at the surface of the wing at various points, claiming without any proof whatsoever that the results are accurate, while pilots use a device that does not use the Bernoulli Effect at all to accurately measure airspeed, which is critical to their survival.
Air has mass. Mass implies momentum. Changes in momentum constitute acceleration. When a closed tube (a pitot tube) is thrust into an airstream, the air that goes into that tube is quickly accelerated into the direction of the pitot tube. That acceleration creates weight, measured by the aneroid wafer. The greater the differential between the speed of the ambient air and the pitot tube, the more acceleration, and the more weight.
That weight compresses the air, so yes, it is denser in the tube than it is in the ambient air. That air would decompress out through the mouth of the tube to match the pressure of the ambient air, except that this is disallowed by the acceleration of more air striking the mouth of the tube.
The constant stream of oncoming air maintains the pressure in the tube, so as the relative speed of the air and tube change, so does the pressure, and so does the pressure reading, which is translated into airspeed.
The Bernoulli Equation was invented to measure blood flow in a blood vessel in the 1700s. It assumed an incompressible fluid flowing through a vessel with parallel walls, neither converging nor diverging.
Einstein ignorantly developed the "Longer Path" theory of lift generation of wings based on a misapplication of the Bernoulli Effect. Orville Wright argued that he was wrong, and Einstein accepted that he was ignorant and should not have posited the theory, but because of hero worship for Einstein's intellectual prowess, engineers really, REALLY want to use the Bernoulli Equation to measure airspeed.
Pressure taps measure pressure. They do not measure airspeed. Models of airspeed at the surface of wings based on data from pressure taps are deeply flawed, and yet they dominate aeronautics.
If the Bernoulli Equation could give you an accurate airspeed, then airplanes would use a device different from a pitot tube with an aneroid wafer to accurately measure airspeed. Engineers proclaim that air is "incompressible" at subsonic speeds, and yet here, inside the pitot tube is compressed air. If it wasn't compressed, the instrument would not measure airspeed.
And yet, it does.

"Engineers proclaim that air is "incompressible" at subsonic speeds" - I've never heard that before.  Where is that from?

"engineers really, REALLY want to use the Bernoulli Equation to measure airspeed." No, we really, really want people with not understating of engineering to stop trying to guess at what we do or want to do.


text:
Throttle friction lock - what is it?
Referring to a general aviation airplane, what is the throttle friction lock and what do I need it for?

It's a device that makes the throttle harder to move. You need it to keep the engine vibrations from slowly moving the throttle away from where you left it during cruise.


text:
What does performance speed refers to? Aircraft performance in general
For example C172R specifications:
Cruise speed:122 knots
What does this mean?
Maximum achievable Indicated airspeed At specific conditions?
Another example - rate of climb: 721 ft/min
For what conditions? So much factors can affect this..

Read the POH - it will show the conditions the speed was measured under.

Cruise, 75% Power at 8500 Feet. . . . . . . . . . . . . . . . . . 122 KNOTS
NOTE
*Speed performance is shown for an airplane equipped with
  speed fairings which increase the speeds by
  approximately 2 knots. There is a corresponding difference
  in range, while all other performance figures are
  unchanged when speed fairings are installed.
The above performance figures are based on airplane weights at 2450
  pounds, standard atmospheric conditions, level, hard-surfaced dry
  runways and no wind. They are calculated values derived from flight
  tests conducted by Cessna Aircraft Company under carefully
  documented conditions and will vary with individual airplanes and
  numerous factors affecting flight performance.

(http://www.airilm.com/wp-content/uploads/172R-POH.pdf)

122 Indicated or true ?


text:
Past depression affect my ability to obtain a Canadian medical certificate?
I'm from Canada, and two years ago I had problems with my wife and decided to divorce. After that, I felt discomfort, and I was diagnosed with depression.
I took an antidepressant Effexor for one year. But another doctor decided to stop my medication because he believed that it was normal to be sad after divorce, and I lm not a depressed person! so I have taken no medication in the last year. I think that the first doctor had made a mistake by giving me this medication.
Should I still be able to qualify for a medical 1 in Canada?

Related: https://aviation.stackexchange.com/questions/27345/will-having-a-history-of-depression-prevent-me-from-flying

This question is different from Will having a history of depression prevent me from flying?, which is asking about the USA. This question is about Canada.

Edited the post and retracted my close vote.

@DanieleProcida While you are right that this question is specifically about Canada and the linked duplicated is about the US, I don't think the answer will be any different. The linked question is answered with ask an AME because every case is different and this cannot be answered by strangers online, so that is what you should do in Canada as well.


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What’s the purpose of the small “hole” in front of wing root on A320-214?

Someone knows what’s the purpose of this little “hole”?

There is a bit of dirt, but not a heat shield.  This means it's an exhaust port for something, but not something super hot, like combustion air (like an APU, which would be a lot bigger anyway).  It's likely an exhaust for a heat exchanger of some kind, possibly a hydraulic fluid cooler (an air conditioning heat exchanger outlet would be larger).

Are you sure it's an A320neo? Other pictures of the plane showing the right side (such as here and here and here) don't show such a feature.

Are you sure that's a picture of an A320NEO?

CUT HERE IN EMERGENCY...pure plane, please don't cut it!

Did you take that picture? If so, can you tell us which airline this was and from where to where you were flying. That should help narrow down the actual aircraft. If it's not your picture, could you add the source to the question? A reverse Google image search did not find anything.

Pretty sure it is an A320 like this one. The Portuguese helped me narrow it down.   https://www.jetphotos.com/photo/8795441

Here is another one. Whatever it is it's not standard: Most 214s don't have one.
https://commons.m.wikimedia.org/wiki/File:Airbus_A320-214,_TAP_Portugal_JP6867739.jpg

I’m sorry for delaying in replying! So, I’m not sure if aircraft is an A320NEO. The aircraft tail number is PR-TYN an A320-214. I think that this “hole” is connected to (Inert Gas System), but I’m not sure.

My company's A320 also have that mystery hole. After a thorough search, it's the vent outlet hole for dumping the forward cargo compt air overboard, through the extraction fan (6HN) and outlet isolation valve (4HN). Detail can be found in cargo ventilation chapter (ATA 21-28-00).
It seems that hole are not installed on every A320. From the SRM (Structural Repair Manual), that hole is surrounded by a doubler and the doubler can only be found on ...... 282 aircraft (data as of today).

SRM = Structural Repair Manual? Can you add it in the answer?

@ROIMaison Yes and added, I thought it was quite a well-known acronym

It leads to a vent in the cargo compartment that Airbus calls "Outlet-Extraction". It must be an option that isn't installed very often.

Hi and welcome to the site! Do you have any references to support your answer? We appreciate that kind of thing.


text:
What is the process to contest a local FAA (FSDO) policy which appears to contradict their guidance?
As the title asks, what is the formal process to contest a local FSDO policy which appears to contradict a FAR or FSIMS (8900.1) guidance?
I have already sent an email asking them to reconsider, to which the inspector reviewed the request with his front line manager and said that they were not going to change it (in spite of what I feel is clear guidance to the contrary).
I then asked for the policy to be reviewed by the local FSDO manager who would be in a position to change it, who also said that the policy would not be changed.
What is the next step?  Elevate it to regional/national?  I remember reading about a formal process where you make a request, and if it gets denied you have 30 days in which to ask for it to be elevated to the next level, but I don't know where I saw that.

While this is a good question, you may get some more specific answers if you list the actual infraction as there may be different elevation methods depending on the situation.

@Dave I intentionally kept it general so that it applies to any situation where a local FSDO policy in in question and to not get caught up in the details of the specific question (FYI, no infraction is involved but rather just a FSDO policy which is causing more work for a lot of operators only in a specific region!)

Are you looking for something like that: CSI? More here.

@mins That looks very promising, although I haven't had time to review it and don't believe it is what I saw in the past.  Thanks!

I think the "30 days" stuff you're remembering may be one or more of the many generic "We have to get back to you within 30 days" requirements the FAA operates under, or the standard "You have 30 days to appeal this decision" for FAA actions/violations (if you want to bring them before an NTSB Administrative Law Judge). As far as I'm aware there's no time limit on escalating a bad FAA interpretation - you just have to live with the FSDO's interpretation until such time as you can get a better one...

The FAA Hierarchy is your typical government bureaucracy - you can escalate your way through it like most others:

Local Level
Your point of contact for all things FAA will typically be a local district office -- for most things you would start with the Flight Standards District Office (FSDO) unless you know specifically that your issue falls into the domain of another office (like Aircraft Certification or Manufacturing).
If you can't work out the issue with the local folks at your FSDO you can try moving laterally and contacting someone from the FAA Safety Team at the FSDO (they may not be able to change anything, but they may be able to help get you a better explanation of why things are being interpreted the way they are, and give you a voice on the inside).
Regional Level
Each district office is contained within a region, and each region has a regional administrator. The regional administrator's office would be your next point of escalation. As with the District Offices contact information is available on the FAA website, and if you are able to convince the regional administrator's office that the district office has made an error and their interpretation is inconsistent with published FAA guidance you may be able to get the local interpretation overruled.
National Level
Your next step would be the appropriate FAA Headquarters office -- for most stuff it would be the Aviation Safety office (AVS), and as mins pointed out you probably want to avail yourself of the Consistency and Standardization Initiative which exists to bring some standardization to what has traditionally been the the inside joke of "There's nothing standard about Flight Standards!" -- If a local FSDO is operating contrary to published guidance in FSMIS or an Advisory Circular these folks can set them right.
There are separate national offices that deal with Air Traffic and Airports which may also be useful contacts if you have incidents with those divisions.
The Big Legal Hammer
When all else fails and you need a legal interpretation to tell the entire FAA "This is what the language in 14CFR means, and this is how it must be interpreted." your point of contact is The Office of the Chief Counsel.
These are the folks you contact if you have a specific question about how to interpret the FARs that you can't answer concretely from an Advisory Circular, or when you have contradictory interpretations from different parts of the FAA (dueling FSDOs) that you can't resolve through the other channels.
An interpretation by the counsel's office is binding on the entire FAA organization, and as you climb the ladder there's an increasing chance that the folks you talk to will involve the lawyers at the counsel's office.

In practice the regional level is often skipped - particularly given the Consistency and Standardization Initiative at the national level: It is often more efficient to escalate directly to a group solely charged with making sure the national standards are consistently applied.
As you climb the bureaucratic ladder the chances of someone involving the FAA's lawyers for a final legal interpretation increases: a motivated employee who thinks they're right may refer the matter to the counsel's office and copy you on the request (or you could get fed up and do the same).
Once you reach the point of an interpretation from the Office of the Chief Counsel you've climbed as high in the bureaucracy as you can - an unfavorable interpretation by the FAA lawyers can only be challenged by bringing in lawyers of your own and hashing it out in court.
Because the US court system has a long established practice of deference to agency interpretation of their own regulations the chances of winning such a case are not good unless the interpretation of the agency is entirely unreasonable or the application can be proven "arbitrary and capricious" based on other similar cases.

If you want to force a policy change, write directly to FAA Chief Councel.  If CC writes an opinion that aligns to your position then the FSDO really can't have their own opinion.  If, however, CC disagrees with you then you're sunk unless you want to take it to court.

Thanks, I considered this but as I mentioned, I believe that there is a formal process to follow in order to elevate it so that we don't have to go directly to the Chief Council (I'm fairly certain that it doesn't need to be elevated quite that far to get a favorable response).

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


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What is 0650// in the metar?
I have this metar, I cannot find anywhere what does 0650// or 1600// or 0250// mean. I tested on multiple decoder websites. They call it unknown identifier.
ESCF 240220Z AUTO 00000KT 0800 0650// R01/P2000N R19/1900U BR OVC035/// 00/00 Q1005
ESCF 240250Z AUTO 10001KT 4100 1600// BR OVC033/// 01/01 Q1005
ESCF 240120Z AUTO 00000KT 0450 0250// BR OVC037/// 00/00 Q1005

This is most likely the minimum visibility (in metres), with the compass direction given as "//" (not possible to determine). A single slash is more common, i.e. 0650/. WMO Publication No. 306 - Manual On Codes - Volume I.1 - Part A, Section A, FM 15 METAR, FM 16 SPECI (emphasis added):

15.6 Groups VVVV VNVNVNVNDv
15.6.1 The group VVVV shall be used to report prevailing visibility.
  ...
15.6.2 Directional variation in visibility
  VNVNVNVNDv
When the horizontal visibility is not the same in different directions and when the minimum visibility is different from the prevailing visibility, and less than 1,500 metres or less than 50% of the prevailing visibility, and less than 5,000 metres, the group VNVNVNVNDv shall also be used to report the minimum visibility and, when possible, its general direction in relation to the aerodrome reference point indicated by reference to one of the eight points of the compass. If the minimum visibility is observed in more than one direction, the Dv shall represent the most operationally significant direction.

TLDR: 0650/ should have been 0650//. 0650 is 650 meters minimum visibility and // is Direction not available.


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During airshows, how common is it to fly armed with (inert?) missiles?
I notices that these three Su-35s were armed with missiles during an airshow. At the 1-min mark, you can clearly see that jet #38 has 2 missiles under each wing as well as a bomb / (spare fuel tank?) and that jet #3 has a missile under at least 1 wing. At 5:25, that jet has 3 missiles. etc.
I assume these are all inert, non-explosive? And used to prove the Super-Flanker is still super-manuverable even when armed?
Is this common practice for all air forces to demonstrate the performance of their best jets even when armed? I've thought of airshows as a demonstration to "the people" where you try to impress them with aerobatics and get them to clap and appreciate how much money is being spent on the air force. Thus, why the weapons?
Of course, if Russia were trying to get Erdogan to switch to the Sukoi family, then you must demo fully armed jets.


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What questions should I expect during an EASA skill test?
I have my skill test in few days. I'm lost on what are the questions that will be asked during the oral part of the atpl skill test.

It would help if you told us what kind of license you're getting... I'm sure the questions are different for private, commercial, ATP, helicopter, sea planes, etc.

That would be great, I'm getting atpl license from Greece.

Goodness, the best not guess, since the amount of information an ATPL candidate must master is just massive. Could be anything from the smallest detail to broad concepts. Anything to make you uncomfortable and see if you are willing to admit that you might not yet know everything, or whether are you the kind of guy who takes the best guess and hopes it's the right one...
For your sake I hope all goes well, and for everybody elses, I hope you know everything needed to become a safe professional airline pilot. Godspeed.

This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review

@dalearn I'd disagree... question is, what to expect; answer is, expect a very broad range. That's an answer, and more reasonable than one that attempts to list all that might be asked.

@dalearn Honestly, I do deeply sympathize with the anxiety of the asker, but I answered just  as honestly, since it would be misleading to give any kind of listing of possible or likely question he or any ATPL student might face. This is the least speculative answer anyone can give. Some questions do not have definitive answers.


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Grand Canyon flying minimum altitudes
I'm looking to visit the Grand Canyon this September. I fly an open-cockpit two-seater gyroplane - a Magni M16. Its ceiling is around 13k' and I'm not wild about trying to reach the top end of that. I'm confused by the SVFR rules on the special Grand Canyon chart and would appreciate some clarification.
If I look at the chart on Skyvector, I can see the allowed corridors and the SVFR area fine. Looks like in the east it's more restricted and for the Native American areas, you have to be above 14.5k' to fly over them (which rules them out for me anyway) but that I can use the Dragon and Fossil Canyon corridors down to 10.5k', which should work. The rest of the area I need to be at least 10k', at least over the Supai and Diamond Creek sectors.
So far so comprehensible. However, in the west, you have to avoid The Sanup tribal area below 8k' but then the rest of the area is either the same or higher than that! Do I have that correct? Looks like I could depart from 1G4 and fly due east over the Sanup area at 8k', then turn north and provided I followed the edge of the Pearce Ferry sector, I could stay at 8k' all the way to the Bar 10 airport 1Z1. Is my interpretation correct?
Thanks for any help and guidance, especially from anyone who's done this in a low power open-cockpit craft.

If your question is "what is the best way to fly over this region with an autogiro?", the title should be formulated reflect it.

It isn't. I provided that just for background info.

Yet, you may rephrase the title so that the subject is clear without opening the question. Don't hesitate to visit the help center for more information

The rules for operations in the Grand Canyon Special Flight Rules Area (SFRA) are governed under 14 CFR Part 93, Subpart U.
Please review these rules here and all applicable GCSFRA NOTAMS prior to departure.  Also perform your performance calculations for the predicted density altitude on the day of your flight to verify your aircraft will be capable of ascending to at least 9500 ft MSL, else it may not be legally or physically possible to make the flight.
For an eastbound flight like that departing 1G4, I’d recommend a takeoff and climb to 9500 ft MSL (eastbound VFR altitude which meets the minimum requirements of both the Pearce Ferry and Diamond Creek sectors of the GCSFRA) prior to overflying the Pearce Ferry sector enroute for 1Z1.  Be advised for both fixed wing and rotor wing aircraft over the northern sections of the Pearce Ferry and Diamond Creek sectors.  Commence your descent into 1Z1 only once clear of the northern edge of the Diamond Creek sector.
Aside from the legal aspects, keep in mind that you are going to be flying over some pretty rugged country out there.  Have a plan of action should an engine failure or other inflight emergency occur enroute, requiring you to set the aircraft down somewhere.

It looks like subpart U just deals with commercial operators. I will however definitely review them, and the NOTAMs and be aware of the density altitude. Thanks for the advice.


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What happens when a GA pilot squawks 7500?
If you were to put code 7500 into your transponder in a small general aviation aircraft, and you were in a rural area with no one around, and you did not have enough fuel to get to any city, what happens? Would they shoot you down? {edit what I meant was if you were being Hijacked and you on purpose sqacked 7500

Here is quite a “what-if” question. Are you asking about inadvertently squawking it or purposely squawking it? If you add more detail to your scenario or state why you are asking it, it may help people answer.

I know a guy who accidentally set squawk 7500 during a flight in a glider. He had turned down the radio, so no one could contact him. At some point, a helicopter was sent up to intercept him. Oblivious to the fact that he was squawking 7500, he just waved at the helicopter pilots as a greeting. After landing, I think he had a few missed phone calls to answer ... but he wasn't shot down!

Who would shoot you down and why? You are in the middle of nowhere. Who are you endangering? ATC would try to contact you to ascertain your actual condition. You may have accidentally put in the wrong code. They would then offer assistance if needed.
If it was a kidnapping situation, all they could do is follow or track you until you land. You are not going to get far. Even if you went to another country close by, you can still be tracked and local authorities alerted.

If you enter a 7500 code in a transponder, I'd expect the authorities would almost certainly intercept you with instructions as to where to proceed to and land.  They have no idea as to your particular situation other than you're transmitting an aircraft hijacking emergency code and they are not going to take chances.  As to using deadly force against you, it is an option, provided you are posing an immediate danger to the public or national security.  The most likely scenario is fighter interception followed by instructions as to where to go and land where ground based law enforcement can take control of the situation.


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Why don't smaller powered airplanes have better lift-to-drag ratios?
Many airliners have LDRs near 20, and it sounds like the electric Eviation Alice may have an LDR of 24 at 240 kts, but many small planes typically have LDRs of only 8-10.  It seems like they could easily increase the ratio with e.g. larger wingspans with higher aspect ratios and save a lot of fuel - why don't they do this? What are the tradeoffs? Are there any commercially successful 4-20 PAX planes that have LDRs over 20?

There are plenty of smaller power planes that achieve those numbers; motorgliders.  And motorgliders with L/Ds in the high teens and low 20s are pretty efficient cruisers.
So the real question is; why aren't all light aircraft made to be like motorgliders?
Well, motorgliders have their disadvantages.  The long wing span is a problem fitting in on the ground.  The long wings are heavy, which limits payload.  The long wings result in slow roll rates and general controlability problems.  So if you are going to live with the hassle of long wings, it has to be really worth the trouble.
And probably the biggest one; the long wings stop being an aerodynamic advantage and become an impediment when you want to cruise at significant margins above speed for best L/D.  This is the case for aircraft that cruise at low altitudes if you want to cruise at 2-3 times max L/D speed efficiently.  In this case, < span = better.  WW2 fighters modified to be optimized for low altitude combat had their wing spans reduced.
Airliners are more like the motorglider, and benefit from the L/D benefits of high aspect ratio because of their operating environment.  They cruise at very high altitudes where indicated airspeeds are low because of the thin air.  At FL400, an airliner might be cruising in the thin air at only 280 kts indicated, maybe only 2X its flaps up stall speed (going 500+ true, but it's the indicated airspeed that matters - if you could stick your hand out the window, you'd feel a 280 kt wind blast, not a 500 kt wind blast).
It's cruising a lot closer to its max L/D speed than the low altitude airplane that flies at 3-4 times its stall speed, and so it benefits from the span and area like a motorglider.  To take that to it's most extreme, you have the Lockheed U2, which needs all that span and area to make it up to 75000 ft (where the indicated airspeed is only about 80 kt going, say, 200 kts true or whatever it is).
You do see some aircraft like the Diamond light twins with quite long aspect ratios for what they are.  This is done mostly for rate of climb. because in a light twin, single engine climb performance is critical, and the high aspect ratio really helps (climbing close to max L/D speed) and that's factored into the balancing act of deciding just how long the wings need to be for good single engine performance without giving up too much controlability or taking on too much weight (helped there by composites) or too much space on the ramp.

I don't understand why bigger AR is an impediment at speeds much greater than max L/D. Are you thinking of transonic effects?

I was thinking mostly of the effects of the increased frontal area of a high AR wing relative to a low AR wing of the same total area.

I would like to see some reference on this claim. I would think the reason fighters have low AR is for maneuvering and weight. High speed aircraft have transonic effects.

@JZYL, higher aspect ratio reduces induced drag at cost of some increase in form drag. Induced drag decreases with square of speed, while form drag increases with square of speed. So high aspect ratio give better max L/D, but at lower speed, while at higher speed the total drag increases due to the higher form drag. Still, A320 has AR 10.3 and $V_y$ around 250 KIAS (depending on weight), while C172 has AR of only 7.32 (lower AR means higher $V_y$), but it's $V_y$ is just 78 KIAS. So there is a lot more in play than just AR.

As far as the frontal area of the wing itself is concerned, if your high AR wing had the same thickness as the low AP one they should have roughly the same equivalent flat plate area.  The problem is you normally can't do that for structural efficiency reasons and are forced to use a thicker section at least at the root, so in practical terms you are stuck with more form drag on the high AR wing compared to a low AR one with the same area because the high AR one has to be more or less almost as thick as the low AR one, at least toward the root.  OR, make the whole thing substantially heavier.

@Jan Why do you say AR increases form drag? In a complete incompressible environment, increasing AR decreases Reynolds number, which decreases form drag.

@JZYL, comparing wings that should generate the same amount of lift increasing AR means increasing the span, which means increasing the cross-section, and the cross-section has largest effect here.

@Jan Again, it would be great to see some reference on this. Otherwise, why would the turboprops have such large aspect ratio

@JZYL, turboprops have larger aspect ratio, because they are designed for lower speed; that matches what I said.

In general you need enough wing to lift your weight at a given speed at the optimal L/D ratio.  Going with a lower area, higher aspect wing should be more efficient in cruise, strength issues aside.  This is the trend in airliners, but a higher aspect wing (without slats and flaps) has a higher stalling speed.  This is why eagles and vultures have broad lower aspect wings for slow turning flight and albatrosses have thin high aspect wings for cruising.  Depends on what you do.

First of all: Scaling laws. The ratio of inertial to viscous forces in the flow around the aircraft becomes larger with bigger size. This is also helped by the generally higher wing loading of lager aircraft which results in higher flight speeds. This ratio is expressed in the Reynolds number Re:
$$Re = \frac{v\cdot l}{\nu}$$
In simpler terms: Friction is relatively lower for larger and faster aircraft.
Next, jet aircraft are more aerodynamically clean than propeller aircraft. Until jets are used for General Aviation aircraft, those will still have piston engines with radiators and propellers. The radiator alone (or the ducting around air cooled engines) can increase drag such that up to 10% of installed power is eaten up by this additional drag. Also, a clever study has identified the additional drag caused by the propeller slipstream on a Luscombe 8b: In that case it was 30% of zero-lift drag!
Last, while airliners are designed to fly at their optimum L/D polar point, GA aircraft are normally flown quite a bit faster which reduces induced drag. In order to enable a high cruising speed, the wing has a lower aspect ratio than that of most airliners. The ever increasing efficiency of turbofan engines has allowed to fit higher aspect ratio wings to airliners such that a Boeing 787 now has an aspect ratio similar to propeller-powered, long-range aircraft like the P-2 Neptune, the Breguet Atlantique or the B-24. Older airliners needed more internal wing volume for fuel storage and their lower aspect ratio also lowered their optimum L/D (which is only 16 in case of the Boeing 747-200, for example).
A well-designed GA aircraft need not be vastly inferior to airliners: While a DHC-2 Beaver amphibian only manages an L/D of 9, a Glasair III achieves a very respectable 19.7.

Lower aspect ratio, given the same surface area, increases Reynolds number and increases induced drag. I don't see why it's more aerodynamically efficient. GA aircraft aren't flying near the Mach regime for subsonic effect to matter.

@JZYL like the way you hang in there, and you're right, for a given area, higher AR is more efficient in cruise.  Shame we have confused simply making the wing smaller also makes it shorter, with the same AR (just look at jet powered cruise missiles) and lowering AR, where strength and turning benefits are greater.  Notice bomber designs are generally higher AR than fighters, back in the "old days".

@JZYL FAR have mandated in the past a minimum speed of 61 knots for light GA aircraft. This requires some wing area which produces a lot of viscous drag in cruise. Induced drag in cruise is almost negligible, so it is preferred to trade a bit of viscous drag (by increasing chord for the same area) for a small increase in induced drag. Also, the structure becomes lighter with lower AR which again helps to achieve those 61 kts.

Alice is a very promising design featuring these efficiency improvements:

Vortex cancelling wing tip props
2 empennage fins instead of 3
Aft main propeller mount
Streamlining of fuselage
Straight high aspect ratio wing
Light weight composite construction

Over many miles, as with airliners,  these improvements offer significant energy savings which translate into cost savings.  But craft such as these are expensive, and must operate for a given amount of time with a given amount of passengers to pay for themselves.
These are different concerns from GA aircraft, which soldier on with their fat Clarke Y or Gottingen airfoils for one very good reason: safety.  These wings have very gentle stalling characteristics that, combined with strong lower aspect "Hershey bar" form, and slower flying speeds, provide a much more reliable and easy to fly experience for recreational pilots.
Polar diagrams at Airfoil Tools show the differences in lift to drag and stalling AOA characteristics of many airfoil types, and make good reading to further understand the trade-offs of gliding efficiency and safety.


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How to determine angle of bank caused by shift in centre of gravity in an aircraft?
I’m trying to figure out the angle of a bank caused by a shift in the centre of gravity of an aircraft. How should I do this?
What avenues should I go down to calculate this? I know that the plane will bank until c.g. is below centre of lift.

this also depends if the aircraft has dihedral stability as well.  I would read up on what makes an aircraft stable in flight to better understand how to "destabilize" it when you wish to change direction.

you may wish to study hang gliders.  Sorry for the tangle here.  Shifting weight to the side will cause a rolling force.  How far it progresses depends on roll stability of aircraft, and, of course, control inputs.  This will be a unique property of each type of aircraft you study.  Your initial thought is correct.  You can learn more by building free flight models and adjusting the weight not only fore and aft, but also side to side (I use pennies).  Good luck with this.

A sideways shift in the center of gravity will give you a rolling moment but not a stabilized bank angle. The rolling moment won't magically stop once a certain bank angle has been reached. What could stabilize the plane is a combination of speed, bank and sideslip, but not all designs will be stable at such an equilibrium.

@ RobertDiGiovanni and original poster-- Riffing on Robert's comment-- in most  modern hang gliders, at LOW angle-of-attack (pilot's CG well forward, airspeed initially high), shifting pilot's weight say 12 inches to one side and keeping it there will result in a rather modest bank angle-- a wild guess would be maybe 15-20 degrees-- while at HIGH angle-of-attack (pilot's CG well forward, airspeed initially low), the same input will cause the glider to "wind up" into a rather steep spiral, eventually reaching a bank angle of over 60 degrees, in which case the airspeed will no longer be "low".

So-- "it depends".  In conventionally-shaped model airplanes with lots of dihedral, I've been surprised how little visible change in the "trimmed" bank angle is obtained by a rather significant weight on one wingtip.  In the hang glider case, we could probably go on for several pages about all the different factors playing a role here, including the way that the dihedral-like effect created by wing sweep is strongly dependent on angle-of-attack, and the way that the way that washout creates a roll torque toward wings-level is also strongly dependent on aoa, but in the opposite direction.

You can't. Aircraft are not stable in roll!
Once there is an offset between the centre of gravity and the lift action line, there will be a rolling moment until the aircraft is banked at 90°, at which points the forces become perpendicular, so they can't generate any moment.
The moment will cause roll acceleration, but as the wing going down will have higher angle of attack, its lift will increase, causing opposing moment that will stabilize the aircraft at some roll rate. Which will slowly decrease as the moment between lift and gravity decreases with the increasing bank. But it will not vanish until 90° bank, so the aircraft keep rolling and end in a graveyard spiral.
If there is some roll-yaw and yaw-roll coupling, and the imbalance is small enough, the side-slip created by the incipient turn will generate enough opposing roll moment to return the plane to a dutch roll instead. But that is an oscillating motion, so you still won't have stable bank angle.

Some aircraft are not stable in roll, but most are.

@MichaelHall, it is not possible to have stability in pure roll. Aircraft do have roll-yaw and yaw-roll coupling that makes small excitations in roll produce dutch roll instead of runway bank, but it usually only works at low bank angles, because otherwise the dutch roll would be rather uncomfortable.

I'm not saying that there isn't some small amount of dutch roll as the aircraft recovers from an upset, but to imply negative stability in roll as the norm is misleading.

@MichaelHall, the dutch roll is the only mechanism that provides any kind of positive stability. Above certain bank angle, the yaw-roll coupling provides a negative stability instead, but that is also yaw-roll coupling. In coordinated flight, roll stability is always neutral.

Ok Jan, I respect that you prefer to keep an air of mystery about you, but it would be helpful if you were to establish your credibility in this matter.  My experience matches what I have been taught; that dihedral and other effects produce positive stability in the roll axis.  I can demonstrate this by banking to 5-10 degrees, centering the ball, and letting go of the controls.  The aircraft will come back to wings level.  If we are just splitting engineering hairs over the contribution of yaw then so be it, but I stand by my assertion that most GA aircraft are stable in roll.

... and if you shift weight from side to side within the fuselage the aircraft will not end up in a graveyard spiral.

Actually, Dutch Roll is a wallowing of a massive aircraft due to the inertial momentum of its side to side roll (top heavy) and insufficient directional stability (usually from a damaged vertical stabilizer).  Also, let's not confuse a graveyard spiral, exacerbated by back pressure on the yoke, from spiral instability designed in to avoid Dutch Roll tendencies.  If one scaled up a Cessna 172 to 747 size, it would be too stable, hence design tradeoffs.  The 747 is low wing, but has dihedral for roll stability.

Given this roll stability now would there be a way to calculate the maximum bank angle before the aircraft rolls back to its position?

@Jim, at some maximum bank angle the aircraft will NOT roll back to upright.  This would vary by aircraft.  Do you have a specific airplane you are interested in?

@MichaelHall the Boeing 747-400. I don't think I was very clear about the maximum bank angle question. Sorry. I meant to ask how can you calculate the threshold bank angle for the aircraft rolling back to it's position.

While it is true that a lateral displacement in the QG will produce some small rolling moment about the longitudinal axis, the effects are minimal and the built in roll stability and trim capability of most aircraft will adequately compensate.
While it is certainly possible to calculate this moment, it is generally a non-issue.  Due to the longer moment arm, wing fuel imbalances have a greater effect than any weight shift within the fuselage.  No weight and balance calculations are performed to determine where the lateral CG lies because in conventional aircraft it will never be out of limits in the way that a fore and aft CG might be.

If shifted the centre of mass was caused by the fuel imbalance would there be a sufficient enough moment arm to calculate the bank?

@Jim, I'm still not sure what you are really trying to accomplish with this question.  Yes, at some point an imbalance could easily exceed the ability to trim out the force, and in an extreme case could exceed the pilot's ability to counter with opposite aileron.  However, it will never just roll to some calculable angle of bank and stop there.  If it rolls on its own it is by definition unstable, and will continue to roll unless countered.  Research positive, neutral, and negative stability and see if that clarifies things.

The weight imbalance is controlled aerodynamicly, so you would figure the (vertical) lift difference of left and right wing based on dihedral angle.  Notice even if it were enough, the plane tilted would start to turn.  If the plane had no dihedral and/or CG below the wing, it would simply roll into a spiral if no corrections were made.  As you overcome the dihedral stability, and then ailerons and rudder with greater imbalance, the plane will crash.  This is also true with fore and aft balance, and why it is so important to have CG in limits.  Aerodynamic control diminishes at lower speed.

@MichaelHall The stability did clarify things. I had the misconception of the fact that the aircraft would roll until the centre of lift was above the centre of mass and stay at that bank.